Lonely Planets

  older times on Venus have been lost.

  Less than 1 billion years ago, the geology of Venus seems to have

  undergone a global pulse of rapid activity and then slowed to a crawl.

  This is disturbing. Venus on the inside should be just like Earth. Earth

  is stable and predictable. Isn’t it? A billion years isn’t all that long ago

  in planetary time. Why didn’t Venus settle down a long time ago, as

  Earth did? Is there something we should know?

  The answer may be that Venus oscillates every half billion years or so

  between spurts of furious global volcanic activity and long spells of rel-

  ative inactivity. Instead of the smoothly running heat engine of plate

  tectonics that our planet enjoys, the internal engine of Venus might run

  in brief dramatic fits of intense activity. Like a motor, seriously in need

  of lubrication, that keeps getting stuck, the global style of Venus might

  be to occasionally lurch into action in massive geological tantrums that

  renew the entire surface all at once, letting a blast of heat out of the

  interior. If this fits-and-starts alternative to Earth’s plate tectonics really

  does occur on Venus, we have to ask, “Why the difference?” Since ther-

  mal evolution, I’ve led you to believe, is controlled by planetary size,

  shouldn’t Venus and Earth have the same overall behavior?

  It may all come back, once again, to location, and the drying of

  Venus by the nearby Sun. The above analogy, of Earth being a well-

  oiled machine and Venus one in need of a lube job, may not be too far

  from the truth. In this case, though, it is water, not oil, providing

  the lubrication. Earth is so soggy that a large portion of the rocks in the

  crust and mantle are hydrated. A layer of water-softened rocks at the

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  electronic edition

  base of Earth’s crust allows the tectonic plates to slide around the sur-

  face of our planet nice and easy, smooth and slow. Models of Earth’s

  plate tectonics, when altered to simulate dry rocks instead of the real

  waterlogged ones, begin to seize up and, instead of running smoothly,

  stop and start like a backfiring jalopy. Dehydrate Earth and it may

  begin to act like Venus.

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  Maybe then, the capacity for Earth-style plate tectonics on Venus

  was lost along with the water in a runaway greenhouse. If this picture is

  accurate, the lightly sleeping monster of Venusian geology may be get-

  ting ready to stir again. Pay attention, because the fireworks could start

  anytime in the next couple of hundred million years.

  Actually, there is good evidence that the monster is not fully asleep.

  There are abundant atmospheric signs of ongoing volcanic activity. The

  mixture of gases in the Venusian air is “out of equilibrium” with the

  minerals at the surface. This means chemicals are primed to react, itch-

  ing to get at each other. When we see such a condition in a planet’s

  atmosphere, it’s a clue that something is up. An atmosphere out of equi-

  librium is like a pile of hungry cats in a roomful of freshly opened cans

  of sardines. The situation is not static. A disequilibrium condition does

  not last long unless some energetic process is actively keeping it that

  way (a constant supply of fresh sardines, for instance, could explain

  why the cats haven’t eaten them all). A disequilibrium mixture of gases,

  left to itself, would rapidly undergo chemical reactions and change into

  a different mix, more in equilibrium. So the atmosphere of Venus has

  not been left to itself. Something is regularly injecting fresh, reactive

  gases into the air. In particular, the amount of SO2 is suspiciously high.

  This is strong circumstantial evidence of currently active volcanoes.

  The most obvious, visible sign of something actively disturbing the

  atmosphere of Venus is the global clouds themselves. In an extreme

  form of acid rain, gases spewing from Venus’s volcanoes are actively

  maintaining the sulfuric clouds and supporting the intense greenhouse

  climate. Without a continuous source of fresh sulfur gases from active

  volcanoes, the clouds of Venus would disappear in a mere 30 million

  years, as sulfur was consumed by reactions with surface rocks. The

  bright clouds of Venus are the smoking gun of active volcanoes on the

  surface in the geologically recent past.

  From this, and the fresh appearance of the largest volcanoes in the

  Magellan radar pictures, many of us believe that Venus today has active

  volcanism. Yet most of the surface seems to have been formed during a

  bygone era when volcanic activity was much more intense. Since we’ve

  found that the current climate and clouds are strongly affected by the

  paltry amount of volcanism occurring now, this leads us to ask what

  the climate of Venus was like 600 million years ago when the global

  volcanic plains were forming, when lava was gushing onto the surface,

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  and greenhouse gases were pouring out of the ground at hundreds of

  times their modern rates.

  Inspired by our mentor Jim Pollack, my collaborator Mark Bullock

  and I have taken on this problem. It’s not easy to tackle because it

  involves combining numerous techniques that have previously been

  handled by several subdisciplines that haven’t always played well

  together. To follow the changing environment of Venus we need to be

  climate modelers, cloud physicists, atmospheric chemists, and volcanol-

  ogists. No one can do all of this, but we’ve had lots of help.*

  First we needed a good climate model, better than any previously

  constructed for Venus. We had to accurately simulate the numerous

  transitions of energy that occur when sunlight reaches Venus, reflecting

  off the clouds, filtering through the atmosphere, warming the surface,

  and reradiating as infrared, which heats the air. But, simulating the heat

  balance in the present atmosphere was only the starting point. We also

  need to be able to change the mix of gases in our model, simulating an

  episode of enhanced volcanic gases, and calculate the changes in sur-

  face temperature, cloud structure, chemistry, and so on. Fortunately, we

  had Jim Pollack to help us design the initial version of our climate

  model.

  Pollack was Carl Sagan’s first grad student at Harvard in the 1960s,

  and he cut his teeth on early climate models of Venus. For several

  decades after that, he oversaw an army of researchers at NASA’s Ames

  Research Center in Silicon Valley and cranked out dozens of important

  papers on an astonishing range of planetary topics. One of his passions

  was climate evolution on Earth-like planets.

  My first job after grad school was as a postdoctoral researcher at

  NASA Ames, with Jim Pollack as my adviser. Jim had a way of cutting

  through to the core of a scientific problem and helping you see clearly

  what needed to be done to solve it.

  The major project Jim and I worked on during my apprenticeship

  with him was the construction of an improved model of
the Venusian

  clouds. For this we used some wonderful infrared snapshots that the

  Galileo spacecraft had taken as it flew close by the night side of Venus

  *As John Lewis says, twisting the “standing on the shoulders of Giants” line of Newton, we’ve been stepping on the ankles of midgets.

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  in February 1990, on the first leg of its wayward six-year journey to

  Jupiter. We used the pattern of the heat leaking unevenly through the

  clouds to nail down their structure and composition.

  During this same period Magellan was in orbit around Venus and

  the first global surface maps were being assembled. The strange crater

  distribution suggested immediately that Venus must have had periods

  of intense volcanic activity. Jim was a global thinker and he encour-

  aged the same in his colleagues—we had wonderful conversations

  about what this strange surface history might have meant for the

  atmosphere, clouds, and climate. He was a rigorous scientist who

  wasn’t afraid to think bold thoughts—he encouraged me to follow my

  ideas, however fantastic, as long as I could back them up with physical

  models.

  Around the time I was finishing my postdoc gig and packing up the

  old Corolla for the move to Colorado, Jim became ill with a rare form

  of cancer. When he became too sick to go to the office, he hooked up

  his first home computer and continued to hold court in cyberspace.

  Like a shark that needs to keep swimming, Jim needed to do science.

  He kept up his input on numerous ongoing projects, barely ever hinting

  at his worsening condition unless directly asked about it, right until his

  untimely death in June 1994 at the age of fifty-five. I’ve kept several of

  my final e-mails from him: strange electronic relics of a great mind and

  a kind soul.

  Starting with the climate model Jim helped us to map out, Mark and

  I have added parts that simulate volcanic emissions of gas to the atmo-

  sphere, chemical reactions between gases in the atmosphere and miner-

  als on the surface, diffusion of gases into the crust, the formation and

  destruction of clouds, and the escape of gases into space.

  What we’ve found is that as the geology of Venus has gone through

  intense oscillations in activity, the climate has followed suit, episodi-

  cally undergoing hundreds of degrees of global cooling, and then

  warming. These extreme temperature changes should have made sur-

  face rocks expand and contract, causing Venusquakes around the

  planet. In fact, in Magellan images we think we see the signs of climatically induced surface wrinkles that formed suddenly all around the

  planet after the epoch of massive volcanic outpourings. Unexpectedly,

  climate modeling may have helped solve some mysteries of Venusian

  surface geology.

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  S U R V I V O R

  When we look into the details of climate and geological evolution on

  Venus, we discover an interconnected maze of ongoing processes, all

  changing and mutually influencing one another. Volcanoes on the surface

  alter the atmosphere and clouds. This changes the climate, which affects

  the surface geology and chemistry and even influences the planet’s interior.

  In turn, these internal changes eventually influence the rate of volcanism.

  In this sense, we’ve been learning that Venus, despite having a surface so

  hot that it glows at night and an atmosphere that has quickly consumed

  every spacecraft we’ve dropped in there, is actually quite Earth-like in some

  profound ways. As extreme and hostile as the environment there seems to

  us, it represents a delicate and subtle balance of ongoing geological, mete-

  orological, and climatic activity. Much planetary exploration involves

  studying dead worlds, surveying places that were once active but have long

  been still, and trying to reconstruct the events of billions of years ago.

  Venus and Earth are in a class of their own. They are both survivors.

  In terms of its geology and climate, Venus, like Earth, is still alive and

  kicking. The rampant disequilibrium and complex climate cycles of the

  type we have found on Venus are often thought to be the hallmarks of

  planets with life. But the idea of life on Venus is not taken seriously

  because of our understandable obsession with water. Later, in chapter

  17, I’ll take a critical look at this consensus conclusion, as I consider a

  new way of thinking about life on planets. I’ll ask whether life, rather

  than being something that happens on planets, might be more properly

  viewed as something that happens to planets.

  Our interconnected “systems” approach to studying Venus has

  helped us develop an arsenal of modeling techniques we are now direct-

  ing toward the general processes of terrestrial planet evolution any-

  where in the galaxy or beyond. We wish to explore the balance of

  chance and determinism that goes into shaping worlds, in order to

  know how much we can safely generalize from the local examples.

  Understanding planetary evolution and the different paths it can take is

  a vital step in comprehending this universe’s potential to make life.

  I N T H E Z O N E

  Earth’s biosphere can be seen as an extension of our oceans, as a capac-

  ity that they have achieved, with ourselves as the semisentient ocean’s

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  primitive thought organs. So, though we run the risk of geocentric nar-

  rowness anytime we define specific criteria for finding life elsewhere,

  naturally we focus on searching for liquid water.

  One key factor in maintaining a liquid water biosphere on a planet,

  over billions of years, is its distance from the Sun, or whatever star it

  may be orbiting. If it is in too close, then, like Venus, its oceans will boil

  away. At too great a distance a water world will at best have icy polar

  caps on the surface.

  When I lived in Tucson, I loved the winding drive up to the observa-

  tory on Mount Lemmon, north of town, where grad students escape

  the summer heat and learn to use large telescopes. As you ascend from

  the hot town to the cool mountaintop, you pass through several distinct

  ecological zones, each existing only within a certain range of tempera-

  ture and moisture and containing distinct species of life. You start off in

  low desert full of giant saguaro cacti, then ascend through mesquite

  grasslands, pine-oak woodlands, and ponderosa pines, and end up in a

  dense, lush forest of tall fir and spruce.

  Does the solar system have a well-defined ecological zone, a range of

  distance from the Sun, where liquid water is stable? If so, the inner and

  outer edges of this zone are slowly drifting away from the Sun as our

  star ages and heats up. Earth is probably safe for another 1 or 2 billion

  years. After that, we will go the way of Venus, our oceans boiling off

  and fleeing back to space as the inner, hot edge of the “habitable zone”

  sweeps outward and leaves our spent planet behind.

  My colleagues and I are now pursuing research to define the habit-

  able zones around other stars. A zon
e of water-based life may be a com-

  mon feature surrounding stars. Even if other, non-water-based chemical

  structures, unknown to us, can support life, it is reasonable to expect,

  given the strong dependence of chemistry on temperature, that each

  kind of life will have a limited temperature range within which it can

  thrive. Such alternate biospheres might have their own habitable zones

  at different distances from the same star. Traveling out through a solar

  system, from hot star to distant, cold edge, you may pass through sev-

  eral ecological zones, just as you do on the drive up Mount Lemmon to

  observe the planets and stars.

  Assuming for now that water is what life needs, then the divergent

  lives of Venus and Earth can teach us where to expect the inner hot

  edge of the habitable zone for water-based life anywhere in the uni-

  verse. What about the outer, cold edge? Is Mars inside or outside the

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  habitable zone? I.e., is there liquid water on the surface? That seems to

  depend on when you look.

  R U S T I N P E A C E

  On the afternoon of August 8, 1996, I was a blissed-out, naked, buoy-

  ant extremophile, floating in the 103-degree waters of Orvis Hot

  Springs in Ridgway, Colorado. Among the relaxed, random chatter of

  other naked floaters I thought I heard a woman casually mention that

  she had just heard on the radio that “they” had discovered life on

  Mars. Her comment did not seem all that remarkable, you must under-

  stand, because this is the kind of thing you hear all the time if you hang

  around hot springs in southwestern Colorado, and after a while you

  learn to calibrate your responses.

  Later that day, however, when we went into town in search of food,

  there it was, screaming from every newspaper box: the banner headline

  “Life Found on Mars!” “Holy shit,” I mumbled, fumbling in my

  pocket for fifty cents. That’ll teach me to hide from e-mail for a few

  days.

  The discovery concerned a meteorite that had been found in

  Antarctica in 1984 and determined to be from Mars. This part wasn’t

  new—we’ve known for over a decade that more than a dozen rocks in

  our meteorite collections come from the Red Planet.

  Now, however, at a press conference held at NASA headquarters in

  Washington, a group of reputable scientists had announced with great